1. Field of The Invention
This invention relates to multiple electrical contactor systems having a control module which transfers between contactors overload information scaled to the respective trip settings of the contactors.
2. Background Information
Contactors are switches which connect medium to large power consuming electrical loads, such as for instance, electric motors, to a commercial power system. When combined with overload protection for a motor, the device is referred to as motor starter. As used here in, the term "contactor" is intended to denote throughout power switches with overload protection for use with motors or other electrical loads.
In a number of applications, multiple contactors are operated in a coordinated manner to perform a desired task. For instance, forward and reverse contactors are used to control the direction of rotation of a 3-phase motor by selectively providing opposite phase rotations. In another application, two pairs of contactors are used to alternately connect a 3-phase motor in a Y configuration for start-up and low speed operation, and to switch to a delta configuration for high speed operation as the motor speeds up.
It has been the practice for certain reduced voltage starter systems to use a stand alone overload relay which monitors the current drawn by the active starter. U.S. Pat. No. 4,819,118 discloses a reversing motor control system which does not require a stand alone overload relay. Each of the contactors in this patent contains a microprocessor which provides overload protection by calculating a thermal profile. The active one of the forward and reverse contactors, referred to as the "talker", continuously supplies its calculated thermal profile to the non-active, "listener", contactor, so that upon reversal of the motor direction the former "listener" becomes active with the current thermal profile.
Commonly owned U.S. patent application Ser. No. 933,282 filed on Aug. 21, 1992 and entitled Digital Modular "Microprocessor Based Electrical Contactor System" discloses a multiple contactor system which incorporates a digital control module which communicates with the individual contactors through serial data links. When control is to be shifted from one contactor to another, the control module requests the current thermal profile from the contactor which has been active and transmits it to the contactor which is to become active.
In both of these current systems which transfer thermal profiles between contactors, the instant value of the thermal profile is transferred as is, without modification. However, in some applications, it is desirable to have different trip levels in the different contactors of a multiple contactor system. This is particularly true in a multi-speed contactor system and can also be true in a reversing contactor system.
There is a need there for an improved multi-contactor system which can transfer thermal overload information between contactors scaled to the requirements of the particular contactors.